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Mk4 Challenge Model - Design

Race-Proven Strength, Balance, and Safety

Challenge cars have left the pits and headed for the starting grid literally thousands of times since 1999. No other replica can even come close to the racing hours and track wins of the FFR Challenge car design. The Challenge car is not just completely dominant, it’s truly the only real race car in the entire replica business.

FFR Chief engineer Jim Schenck has been the steward of this car from it’s inception. Jim is an accomplished pro roadracer in his own right, and has earned countless race wins in almost 15 years of competitive driving.

Let’s get a close look at each of the parts that make this design so dominant.

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    Frame / Chassis

    Full Metal Jacket

    The modified roadster frame enjoys a serious cage thanks to the NASA and SCCA guidelines for open GT-1 cars. The differences in the frame and the tremendous amount of additional tubing mean that street car features like doors that are hinged have been replaced by NASCAR style side impact tubes. Frame mods accommodate mounting and shielding of the racing fuel cell, intrusion plates and racing seats.The chassis is about 15% more rigid than the Mk4 roadster. Most of the additional steel is for safety, rather than chassis rigidity. There is no backbone section to the frame, but the full race cage contributes more than that in added rigidity. Additionally, the fact that the cage is welded along with the frame means that the body needs to be cut along a line from the rear cockpit opening to the back of the roll bar legs.

    We send along a carbon fiber cosmetic cover for this area. The FFR 3-link suspension is the most optimized race suspension we sell, and Challenge frames come ready to use that suspension. Owing to the increased amount of steel in the Challenge car chassis, completed vehicles typically weigh around 100 lbs more than standard roadsters.

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    Chassis Assembly

    More of the same.

    The Challenge car chassis is made by the same crew and with the same materials as the Mk4 street car chassis, but with more tubing.

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    The Sub-Frame

    Built like a Nextel Cup Car without the Sponsors

    The Challenge car chassis is built on the strong foundation of our world class Mk4 frame. Connected to that beautiful tube steel frame is an array of aluminum panels bonded and riveted in place like any good race car should be. Aluminum panels make-up the car’s sub-frame and liners. The entire cockpit, floors, side walls, firewall, dash, nose/radiator area, trunk and foot box are T6061 aluminum panels. The panels were designed using CAD/CAM technologies and incorporate the superior construction methods of modern auto racing and aerospace industries.

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    Composite Body

    It’s 1965 all over again… only better.

    The Factory Five Challenge model comes with the same hand-laid vinylester composite body as the Mk4. The doors are single panel (unlike the double-lined and hinged doors of the Mk4), and have pre-drilled mounting holes and positioned hood. Lamination thickness is 3/16” all around with .015” coat of gloss red gel coat.

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    Body Assembly

    Right Materials, Best Tools, Good People

    The Challenge Series Racer body and body panels assembly comes in a red gel-coat that is intended to be painted BUT can be gel-coated and polished for a quick 50/50 finish (looks good at 50 feet going 50 MPH).

    We sell replacement body shells and panels at a reasonable price to keep the cost of racing and the inevitable contact affordable.

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    Suspension & Brakes

    Races are won in the turns and on the binders

    The strong Challenge chassis is complemented by an outstanding and proven suspension and the ability to use a wide array of excellent brake packages. Suspension design and engineering is an extremely challenging endeavor. Over the years we’ve found what works best in the real world of racing. You can’t get lucky and you can’t say it works unless you prove it. The only way to prove it is at a track and under the most extreme circumstances over many, many years.

    Our engineering team has paid it’s dues.

    Front Suspension
    Up front, the Factory Five Racing Challenge kit comes with a fully independent tubular front suspension with unequal length upper and lower a-arms and high-end Koni brand single-adjustable coil-over shocks (say that three times). Our car has earned huge praise and recognition for its suspension which delivers razor sharp handling and a comfortable ride.

    Unlike the Mk4 kit whereby customers can use the front lower control arms from the Ford Mustang, the challenge kit includes the optional tubular lower control arm. The front suspension components are well engineered and bolt-on with ease.

    For brakes the world is your oyster. Spec legal cars in the FFR-NASA Challenge series must wear Mustang Cobra brakes, but open classers and non-spec racers can run the optional Wilwood road race brakes.

    With the Challenge kit, we supply the coil-over shocks, coil springs, custom mounts, spindle brackets, and upper & lower A-arms with ball joints and fasteners, all ready to bolt together.

    Rear Suspension, FFR 3-Link for Pure Performance
    The Challenge kit is set-up to utilize the 3-link suspension. The 3-link suspension is a great example of how racing and product development go hand-in-hand. After the inaugural season racing our Challenge Series cars in the Mid-Atlantic and West Coast series in 2000, we found the standard four link suspensions had a tendency to oversteer (similar to Mustangs) when pushed to ten-tenths competitive driving. Our engineers began working with customers, racers, and the team at the Bondurant School to develop a more neutral, racer-friendly suspension.

    After more than six years of racing action and thousands of hours of brutal track testing, the FFR 3-link is recognized as a proven and worthy rear race suspension. It works wonderfully, aiding hook-up, eliminating bind and provides an amazingly predictable ride whether you’re at the track or on the street.

    The 3-link suspension is designed to use the Ford 8.8” rear axle and includes a steel framework and panhard bar package that includes high-end Koni shocks.

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    Running Gear

    Designed for Ford Parts.


    In the early days most challenge car were built with spec Mustang running gear (engines/trans/rears). Nowadays with the proliferation of not only spec legal series cars, but monster open track customs, we can only tell you what fits (with respect to engines etc)… we can’t tell you what to use!

    Whether you are racing in the FFR-NASA Challenge series or running an open class racer, you still have to select an engine and transmission to use with this car. We’ve built cars for our racing programs using the full range of Ford small-block and big-block engines. The aftermarket is alive with parts and services related to this category. Additionally, today’s modern running gear provides reliable performance for less money.


    The challenge kit is ready to accept the full line of Ford and many aftermarket transmissions. While there is plenty of room in the transmission tunnel for older set-ups, we still prefer sticking with the newer running gear for affordability and durability. The stock Mustang transmission is either a T-5, T-45 or 3650. We have run T-5’s in our race cars for years with excellent results. Most racers choose the Tremec TKO transmission and upgraded heavy duty Ford Racing clutch.

    Rear Ends and Gearing

    The FFR 3-link suspension requires that you buy an 8.8” solid axle rear end. 1987-2004 Mustang GT’s were equipped with a few different gear ratios. The majority of the cars have a 2.73 ratio. There were optional performance rears that used a 3.08 ratio and many cars were built with automatic transmissions and a fun 3.27 ratio. Keeping in mind that the T-5 is an overdrive transmission, you have some space to improve the gear ratio with aftermarket parts without compromising the street manners of the car. The most common gear sets used on the track are 3.27’s.

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    Pure Function at Speed.

    The interior of the Challenge car is all about the business at hand. No carpeting or street seats here. You get all the functional interior pieces and trim that comes with the roadster, except for items that are not needed by racers. The Challenge kit includes a competition seat for driver, leather steering wheel, competition gauges, small competition windscreen, standard shifter and boot, dash, mirrors and Simpson harnesses are also included.

    Sitting in the car you’ll find as large a cockpit that you have to step into (no hinges on doors). Following a careful entry and exit, you’ll find great race-car visibility all-around and an office that is well suited to the serious job at hand.

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    More than 50/50

    While it’s true that most folks will only see you as you blur past them from 50 feet at more than 50 mph, a good number of people will look at your car up close in the pits. Just because this is a dedicated race car doesn’t mean the small trim doesn’t matter.

    The Challenge car uses almost all the same high-quality trim items as the Mk4 roadster.

    Windshield, Lighting, and Mirrors

    The Challenge kit comes with a competition windscreen (the full framed chrome-on-brass windshield can be purchased as an option). The kit comes with original British style lighting and fasteners. These include the expensive Lucas twin filament red tail-lamps, amber turn signals, license plate light (some guys register their challenge cars!), and headlamps. The original chrome “Bullet style” racing mirror comes in the kit along with the classic pedestal mount rear view mirror.

    Trim Accessories

    Exterior trim includes the original chrome latches and handles (L-shaped for hood and T-handle for trunk all keyed alike), side louvers, front and rear quick-jack bumpers w/fasteners, and competition hood pins. A high quality hood hinge is an option.
    The three-point roll bar of the roadster is replaced by the welded full cage of the challenge chassis.

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    The Hard Numbers

    Configuration – Pushrod 16V V-8 iron block aluminum heads
    Displacement – 331 CID / 4942 cc
    Bore and Stroke – 4.03×3.25 in.
    Compression ratio – 10.0:1
    Induction – Multi-port Fuel Injection
    Maximum Horsepower – 425 @ 6000 rpm
    Maximum Torque – 380 @ 5200 rpm
    Redline – 6250 rpm

    Transmission – 5 speed manual
    Final drive ratio – 3.27:1 limited slip
    Clutch – 10.5 inch single disc

    Front suspension type – Independent double arm
    Rear suspension type – Solid Rear Axle 3 link with Panhard Bar
    Shock-Springs – Koni coil over with ride height adjustment
    Steering – Power assisted rack and pinion 2.6 turns lock-lock
    Brakes – Vented 13/11.65 fisc f/r disc brakes
    Wheels – 18×9 FR Cast aluminum
    Tires – Bridgestone 245/40/18 front 275/35/18 rear

    Weight – 2250 lbs.
    Weight Distribution F/R – 46.2/53.8 (%)
    Wheelbase – 90 in.
    Overall length – 158 in.
    Front Track – 58.5 in.
    Rear Track – 58.5 in.
    Overall height – 47.0 in.
    Overall width – 70.5 in.
    Ground clearance – 3.5 in.
    Fuel Capacity – 22 Gallons

    Layout – Front/mid engine RWD Roadster
    Frame – Tubular space frame with integrated backbone
    Roll cage – NASA Approved 1.5 in DOM tubing
    Sub-structure – Aluminum panels riveted and bonded to frame
    Restraint System – 5 point Simpson Harness

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    Dominate any Track.

    A Factory Five Challenge Car was the very first entry into the 25 Hours of Thunderhill endurance race and Challenge Cars have started and finished every single 25 Hours since.

    Nostalgia is great… but modern beats vintage every time. The fastest original car to circle the full course at Virginia International Raceway has done it in a record 2 minutes, 14 seconds. FFR Challenge cars have run 2:05 lap times with modest 350 HP engines. The legend is not even close.

    If you campaign a Challenge car you will own your local track and consistently run with the fastest cars. Since these cars are used primarily on roadcourses, let’s compare modesty powered Challenge cars to some of the fastest cars there are at tracks around the country.

    FFR Challenge cars have won the Charlie Gibson 300 Enduro numerous times. Challenge cars have completed the grueling 25 hours of Thunderhill for the past four years, earning a third place in class and setting the fourth fastest lap time of all cars in 2004.

    A Factory Five Challenge car ran the Fastest Time of the Day at the 2006 Mid America Shelby Meet against a full field of vintage Mustangs and other vintage cars.

    A Factory Five Challenge car runs 1-2 seconds per lap faster than a 2005 Z06 Corvette at the Bondurant School Maricopa Track.

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    Racing is not for Everyone… But Safety is.

    The Challenge Series cars benefit from the following safety features:

    – Complete NASA Approved competition roll cage with side impact bars, mounting for intrusion plates, footbox steel tubing protection and cockpit steel surround structure.
    – Low vehicle weight, excellent weight distribution, and low vehicle center of gravity
    – 3″ Simpson brand, full racing H-harness for driver and passenger.
    – Collapsible steering colum
    – Driveshaft safety bars/
    – Front and rear energy absorbing crush zones.
    – Energy absorbing tubular steel frame and aluminum chassis panels (not fiberglass).
    – Manufacturing control that maintain high degree of consistency in welding and fabrication.

    Vintage components that are individually FMVSS compliant (ie. lighting, seating material, safety glass etc)
    In the world of hot rods, customs and kits we stand miles above the crowd and it’s in great part due to our steadfast commitment to racing development and safety. It’s also because all of us share a deep love and concern for all the people who may ride in a Factory Five. That includes our families, friends and customers.